An optical disk is an example of optical information recording media from which information is reproduced by using a laser beam. Optical disks are characterized as having a large capacity and are used broadly as media for distributing/storing images, music, or information in computers.
A capacity of an optical disk is determined depending on the size of marks to be recorded. That is, the smaller the marks to be recorded, the larger the capacity can be. The size of the recorded marks basically depends on converging spot size of laser beams used for reproducing information. That is, with a smaller spot size, still denser information can be reproduced without an error. The size of the spot at which laser beams are converged by an objective lens has a limited expanse, having the laser beams not converged at a single point even at the focal point thereof because of a diffraction effect of the light. This is generally referred to as a diffraction limit, which is a limit of the mark length that can be reproduced by λ/(4NA), provided that a laser beam wavelength is λ, and the numerical aperture of the objective lens is NA.
For example, the reproduction limit of the mark length in an optical system of λ=405 nm and NA=0.85 is 119 nm, and the mark in the length equal to or shorter than 119 nm can not be read out accurately. In order to increase the capacity of the optical disk, the wavelength of the laser beams may be shortened or NA of the objective lens may be increased. However, when the wavelength of the laser beams is set to be shorter than 405 nm, it is difficult to manufacture optical components with a short wavelength and practical transmittance. Further, when NA of the objective lens is set to be larger than 0.85, it is difficult to manufacture a special objective lens with high NA. In addition, there is also such an issue of safety that it becomes highly possible for the objective lens and the optical disk to have a collision because the distance between the objective lens and the disk surface becomes short.
A medium super resolution technique is known as a technique for improving the reproduction resolution power by exceeding the diffraction limit. The medium super resolution uses a super resolution film whose properties such as the optical characteristic and the magnetic characteristic are changed nonlinearly depending on the temperatures or light intensities. Described herein by referring to FIG. 9 is a case where a super resolution film whose transmittance deteriorates at a certain temperature or higher, which is depicted in Patent Document 1, for example, is laid over a recording layer.
FIG. 9 is a fragmentary enlarged view of a single track taken out from recorded marks that are recorded along a spiral track on a recording layer of an optical disk. For simplification, only short marks are illustrated as recorded marks 24. A laser beam passed through the objective lens is irradiated on the recording layer as a converging spot 20. While the temperature is increased by irradiation of the laser beam, the converging spot 20 and an area 21 where the temperature becomes increased are not consistent because of revolving action of the optical disk. Thus, there are a high temperature area 22 and a low temperature area 23 mixed within the converging spot 20. Since the transmittance of the super resolution film laid over the recording layer becomes deteriorated in accordance with a temperature increase, the high temperature area 22 comes to have an effect of masking reproduction of information from the recording layer, and only the low temperature area 23 functions as an aperture part for reproducing the information on the recording layer. As a result, the size of the effective aperture that contributes to reproduction can be made smaller than the size of the converging spot that is determined depending on the diffraction limit. Therefore, it becomes possible to reproduce information of the minute recorded marks 24 that are smaller than the reproduction limit. As in this case, a super resolution reproduction method, which forms an effective aperture in the front side of the traveling direction of the converging spot by having the high temperature area as an optical mask, is referred to as FAD (Front Aperture Detection) method.
Further, Patent Document 2 and Patent Document 3 depict an optical reproducing device that is capable of constantly keeping the laser power to the optimum state by utilizing an amplitude ratio of reproduction signals with two different kinds of mark lengths that are recorded on an optical disk. In this optical reproducing device, the size of the effective aperture in the super resolution reproduction is kept as constant at all times to decrease a bit error rate by controlling the laser power such that the ratio of the reproduction signal amplitude becomes close to a prescribed value. With Patent Document 2, it is necessary to embed a special mark pattern by periodically providing an area used for controlling the laser power within a data recording area for obtaining the reproduction signal amplitudes of two kinds of mark lengths. Thus, the area for recording information data is decreased. In the meantime, with Patent Document 3, mark patterns of two kinds of mark lengths are detected from the mark patterns of information data recorded in a data recording area by matching. Thus, it is unnecessary to additionally provide an area used for controlling the laser power, so that the data recording area is not decreased.
Patent Document 1: Japanese Unexamined Patent Publication H7-262617
Patent Document 2: Japanese Unexamined Patent Publication H8-63817
Patent Document 3: Japanese Unexamined Patent Publication 2002-260308